1. Field of the Invention
The present invention relates to an internal combustion engine with a dual turbocharger system having a first turbocharger and a second turbocharger wherein air is charged to the engine by a single turbocharger at low engine speeds and is charged by both turbochargers at high engine speeds.
2. Description of the Prior Art
Internal combustion engines with a dual turbocharger system are known by, for example, Japanese Patent Publications SHO 61-112734 and SHO 59-145328, and Japanese Utility Model Publication SHO 62-16729.
As illustrated in FIG. 10 identified as Prior Art, a conventional turbocharged internal combustion engine 1' is provided with a first turbocharger 7' and a second turbocharger 8' having turbines 7a' and 8a', respectively, and compressors 7b' and 8b', respectively. In an exhaust conduit 20b' downstream of the turbine 8a' of the second turbocharger 8', an exhaust switching valve 17' is installed. Similarly, in an intake conduit 14b' downstream of the compressor 8b' of the second turbocharger 8', an intake switching valve 18' is installed. Further, in an intake bypass conduit 13' bypassing the compressor 8b' of the second turbocharger 8', an intake bypass valve 33' is installed. The timings of opening and closing of these valves 18', 33' and 17' are illustrated in FIG. 11 identified as Prior Art. As seen from FIG. 11, the exhaust switching valve 17' is closed at low engine speeds and is opened at high engine speeds. At medium engine speeds, the exhaust switching valve 17' is partially opened to operate the second turbocharger 8' so that a smooth transition from "one-turbocharger-operation" to "two-turbocharger-operation" is obtained.
However, there are some problems in the above-described conventional turbocharged engine.
First, the second turbocharger operation control by the partial opening of the exhaust switching valve 17' at medium engine speeds is relatively rough, because an exact flow control using a butterfly valve of the exhaust switching valve can not be expected in a case where the bore diameter of the exhaust conduit is large, for example, about 50 mm. As a result, a shock takes place during the transition from "one-turbocharger-operation" to "two-turbocharger-operation".
Second, since an actuator (not shown) for operating the exhaust switching valve must be capable of moving the valve to two positions, a full opening position and a partial opening position, the actuator has to comprise a double diaphragm actuator. As a result, the actuator is necessarily of a great size and it is difficult to mount it in a limited space near the second turbocharger.
In developing a structure for overcoming the above-described problems, it is preferable to locate valve means at a portion of the exhaust conduit 20b' downstream of the turbine 8a' of the turbocharger 8', because an exhaust gas temperature is decreased by about 100.degree. C. at the turbocharger 8' and it becomes far easier to protect the valve means from high temperatures.